What causes the screw modifier to fail and create black 2d slice from a bezier curve?
I suspect you've scaled the curve to 0 in one dimension or another, in Object mode, to flatten it. If the object carries that transform, it still will, after being modified. Ctrl A &gt; Apply the scale of your curve.
Think twice, every time you scale in Object mode during modelling. Prefer to do it in Edit mode.